A wiring substrate in which substrates are stacked in layers is known. For example, a wiring substrate where substrate elements each including a base film, wiring lines, and a cover film are stacked in layers and the wiring lines of the substrate elements are interconnected via conductive paste is known.
According to this wiring substrate, the base film and the cover film are formed of a flexible polyimide resin, and the cover film becomes adhesive when heated. (See, for example, Japanese Laid-open Patent Publication No. 2002-158446.)